problem with xlibs?

["Egor Tur" <worldeb@ukr.net>]

Hi folk.
I have this when compile some programme:

/usr/X11R6/lib/libX11.a(XlibInt.o): In function `_XEventsQueued':
XlibInt.o(.text+0x76c): undefined reference to `pthread_equal'
XlibInt.o(.text+0x786): undefined reference to `pthread_equal'
XlibInt.o(.text+0x7a1): undefined reference to `pthread_equal'
XlibInt.o(.text+0x7ba): undefined reference to `pthread_equal'
XlibInt.o(.text+0x7d7): undefined reference to `pthread_equal'
/usr/X11R6/lib/libX11.a(XlibInt.o)(.text+0xb1c): more undefined references to `pthread_equal' follow
collect2: ld returned 1 exit status
make[1]: *** [ds9] Error 1
make[1]: Leaving directory `/home/iraf/saoimagenew/saods9/ds9'
make: *** [ds9] Error 2

 What is this? problem with libX11? What do? Upgrade xlibs-dev?
dpkg -l xlibs-dev
ii  xlibs-dev      4.2.1-4        X Window System client library development

Thanx.